This invention relates to electrostatic printing equipment for printing on a recording medium using stylus electrodes, and in more detail, to the so-called direct imaging method which simultaneously performs the latent image forming process and the developing process on a recording medium.
For years, the recording method known as the direct imaging method has been used, wherein the latent image forming process and the developing process were isolated. The structure and recording principle in this direct imaging method are explained in accordance with FIG. 1. The recording electrode 1, wherein a plurality of stylus electrodes are implanted, and the cylindrical back electrode 2 are provided face-to-face with a specified narrow gap between them. The recording medium sheet 3 contacts the recording electrode 1 in said narrow gap. Meanwhile, a conductive magnetic toner 5 is applied to the surface of the back electrode 2 by a magnetic field produced by a rotating magnet 4 and other means. The rotating magnet 4, the cylindrical back electrode 2, and the toner 5 form a magnetic brush forming means. The chip end of the magnetic brush, i.e., the toner 5, is placed in contact with the recording medium sheet 3. In this condition, a voltage corresponding to the image signal is supplied to said recording electrode 1 from the power supply 6 and the back electrode 2 is grounded in terms of voltage or receives a backward bias voltage. Charges are thereby imparted to the toner 5 which coats the recording medium 3 by being attracted by the electrical field of the recording electrode 1. Simultaneously, when the recording medium 3 is moved at a constant rate in the direction indicated by the arrow mark "a", the toner image corresponding to the image signal can be obtained on the surface of recording medium 3.
In the above-described existing technique, a direct recording medium using paper as the reocrding medium 3 is also known. This method is disclosed, for example, in U.S. Pat. No. 3,816,840. This method, is excellent for providing reduced size and simplified operation, but also has the following disadvantages.
(1) Since the resistance value of paper is as low as 10.sup.10 to 10.sup.11 ohms cm, the electrical field due to the recording electrode 1 spreads, therefore, not allowing the resolution to be increased. PA1 (2) Since the dielectric coefficient of paper is as small as 1.2 to 2.5 and its capacity is also small, a high recording voltage is required. PA1 (3) Recording quality may easily change due to a large influence of external humidity.
In order to compensate for the above-mentioned disadvantages, the paper thickness is reduced to 40 to 60 m, or special processing must be performed on the paper. Any of these measures inevitably restricts the kind and material of paper, and ordinary paper cannot be used.
Thus, the assignee of the present invention has disclosed in Japanese patent application Nos. 55-64840 and 55-64841, both laid open on Dec. 11, 1981, a method wherein an insulating film having a relatively high resistance of 10.sup.12 to 10.sup.16 ohms.cm is used and a toner image is formed thereon and then duplicated upon ordinary paper. The structure of recording equipment employing this method will now be explained using FIG. 2.
The recording medium 3, consisting of the insulating film formed like a belt, is rotated at a constant speed by the cylinders 8, 9 and 10. The recording electrode 1 is provided at the inside of this belt shaped recording medium 3 and is in close contact with it. In addition, the magnetic toner 5 which is transferred by the rotating magnetic roller 4 is prepared on the side facing the recording electrode 1 via the recording medium 3. The developing equipment 11 uses a back electrode 2 as the sleeve of the magnetic roller 4.
Now, in light of the method explained by reference to FIG. 1, after the toner image is formed on the recording medium 3, the recording medium 3 is rotated and the recording paper 12 is carried parallel to the recording medium 3 by the grounded cylinder 9, and said toner image is duplicated on the recording paper 12 from the rear side using the transfer corona 13 or a transfer roller. Thereafter, the toner image is fixed to the recording paper by means of the fixing roller 14. On the other hand, the recording medium 3 is further rotated and the remaining toner is removed, after the transfer of the image, by means of the cleaning blade 15, and collects in the toner retainer 20. The remaining charges due to the transfer operation of transfer corona 13 on the recording medium 3 are erased by the AC preclean corona 16, to allow repeated recording.
The recording system of this type is capable of using high resistance and high dielectric coefficient film as the recording medium 3 and, therefore, is capable of obtaining a comparatively high quality image from the point of view of resolution while using a low recording voltage. In addition, ordinary paper can also be used as the recording paper.
In this prior invention, the toner 5 is coated on the insulating film and the toner 5 is maintained by a fixing force or friction force. Resultingly, it is always accompanied by the following problem, that is, if the fixing force of the toner 5 is insufficient, the toner disappears from the film by means of the magnetic force of the rotating magnet 4. Moreover, as is described later, when executing a matrix drive between the recording electrode 1 and the back electrode 2, if the fixing force of toner is sufficient, the toner is fixed only with a low voltage, the toner is coated even at the half selected points and resultingly the printing quality is degraded. Moreover, when the resistance value of the toner 5 is low, if a voltage applied to the recording electrode 1 and the back electrode 2 is not continued while the recording medium 3 moves on the recording electrode 1, the charges imparted to the toner are lost and, resultingly, the toner cannot be fixed to the recording medium 3, thus making the matrix drive impossible.